Detecting and indicating device



Oct. 22, 1935. H. A. WHEELER DETECTING AND INDICATING DEVICE OriginalFiled April 1, 1951 4 Sheets-Sheet l g i K m E WE a 5 kw N N .D vs TINVENTOR HAROLD A.WHEEL El? Q BY v ATTORNEYS Oct. 22, 1935. H. A.WHEELER DETECTING AND INDICATING DEVICE 4 Sheets-Sheet 2 priginal FiledApril 1, 1931 llll INPUT 5 H w 1/ 0 H M. u f. n 6 H H V I H A ull It I:I: I: I: I: I4 0 0 20 2 I WPGQQSEQ U NW R o .M/ m m w M I? m 4 J 6HAROLD ,4. WHEELER BY I vvwa/ ATTO R N EY5 Oct. 22, 1935. H. A. WHEELER2,018,540

ma'rncwme AND mvxcwrme mavxcn Original Filed April 1, 1931 4Sheets-Sheet 3 lNVENTOR 'l/ARGLD A WHEELL'W ATTORNEY6 Oct. 22, 1935. H.A. WHEELER 2,018,540

DETECTING AND INDICATING DEVICE Original Filed-April 1, 1931 4Sheets-Sheet 4 ay/0C,

INVENTOR HAROLD AMI/EH58 ATTORNEYS Patented Oct. 22, 1935 Harold A.Wheeler, Grmt Neck, Long Island,-N. Y., H

assignor to Hazeltine Corporation, a commtion of'Dela-ware Originalapplication April 1, 1931, Serial No. 526,857. Divided and thisapplication February 11, 1934, Serial No. 111,791. In Great Brl April 1,1931 4 Claims.

This invention relates to signal detection and more particularly torectifying or detecting arrangements adapted to rectify radio or carrierwave signals.

This application is a division of my copending application Serial No.526,857, filed April 1, 1931, now Patent No. 1,951,685.

The detectors contemplated herein are of a type in which the rectifiedresponse is directly proportional to the amplitude of the appliedcarrier wave.

In its broadest aspect, the invention comprises a rectifying elementhaving in series therewith a capacity across which a modulation oraudiofrequency component is developed, and having in shunt therewith ahigh impedance'through which the direct current component of therectified response flows.

,A feature of the operation of detectors of the type of this inventionis that the rectifying action is effected by the peaks of the signalwaves; hence they will be referred to as peak detectors.

Advantages derived from the use of such detecting devices are: x

(1) The substantially direct proportionality or linear relation, betweenthe signal voltage and the direct current component of the rectifiedvoltage or current;

(2) The undistorted reproduction in the recti-. fied output of theenvelope of a modulated carrier signal input;

(3) The absence of undesirable overload limitations;

(4) The simplicity of operation, requirin'gno polarizing voltages; and

(5) Improved circuit arrangements by means of which the detector is madeto respond alike to rapidly and slowly modulated carrier waves.

Detecting devices characterized by the above noted advantages areparticularly well-adapted for detecting the signals at the output of acarrier-frequency amplifier which is provided with an automaticamplification regulating arrangement for maintaining substantiallyconstant signal strength at the amplifier output; Such a combination ofa carrier-frequency amplifier, a diode type detector and an automaticamplification controlling arrangement is described in my said copendingoriginal application Serial No. 203,879, and is claimed in my UnitedStates Letters Patent No. 1,879,863 which is a division of mysaid'application Serial No. 203,879. The combination in a receivingsystem of an automatic volume control and a detector linear" over theapplied carrier signal."

range permitted by the automatic volume control, is described andclaimed in my co-pending application Serial No. 630,739, filed August27, 1932 as aqgivision of my said application Serial No. 203,- s.

By virtue of the use of a linear type detector, the carrier wave can bemaintained at a substantially constant level at the output of theamphfier, and the modulation or audio-frequency, component of therectified response hasthe same wave form as the modulation component ofthe While some of the present improvements; are applicableto detectorshaving three or more electrodes, the invention will be described withref- 1 erence to the diode detector, which has the merit of simplicity;j

An important application of the invention is a vacuum tube voltmeterwhich'comprises a peak" detector, a direct current'amplifier and acurrent responsive indicator which may becalibrated to measure directlythe voltage applied to the voltmeter. This arrangementwill behereinafter described in detail.

' An important feature of the invention is a 25 filtering systemassociated with the rectifier and so arranged that the direct currentcomponent and modulation current component of the rectified response maybe separated, while at the same time, the load impedance applied totherectifier a is substantially the same for both of said'rectifiedcomponents.

. Another feature of the invention is" the use as a diode of a vacuumtube having more than tw electrodes. In one of these arrangements a pair35 of inner electrodes are utilized as the electrodes of. the diode andan outer electrode is used as a shield between the inner electrodes andthe surrounding space.

Fig. 1 is a circuit diagram of a complete radio 0 receiver whichincludes parts of vthe present in vention, and comprises a three-stageradio-frequency amplifier followed by a rectifier, a twostageaudio-frequency amplifier, and a loud speaker or other suitableindicating device;

Fig. 2 shows graphically a comparison between the performance of thetwo-electrode valve or rectifier, and that of the ordinarythree-electrode detector? Fig. 3 is a fragmentary diagram of thedetector of Fig.1. s Fig. '4 shows graphically thebehavior of the diodedetector of the type of this invention;

Fig. 5 a circuit diagram-of a vacuum-tube voltmeter comprising a diodedetector and a direct current amplifier:

Fig. 6 shows the calibration curves of the voltmeter of Fig. 5;

Fig.7isapartialcircuitdiagramofaradio receiver having a diode detectorwhich receives the output of an auto-transformer-coupled untunedradio-frequency amplifier stage;

Fig. 8 is a partial circuit diagram of a radio receiver having a diodedetector which receives the output of a transformer-coupled untunedradio-frequency amplifier stage;

Fig. 9 is a partial circuit diagram of a radio receiver having a diodedetector receiving the output of a tuned radio-frequency amplifierstage; and 1 Figs. 10a to 10h are fragmentary diagrams of detectorsshowing various types of output networks which are equivalent to a pureresistance load insofar as.audio-frequency and direct current reactionsare concerned.

The terms rectifier" and "detector are each used in this specificationto denote devices which produce direct current or pulsatinguni-directional current from alternating current. The

' terms "rectifying" and converting" are used to describe the process ofobtaining direct current or pulsating uni-directional current fromalternating current. No distinction is made between batteries and otherpower supply devices, which,

it is well understood, can be used with substantially identicalresults.- It is also well known that vacuum-tube cathodes may bedirectly heated filaments or indirectly heated cathodes, with similarresults.

Referring in detail to Fig. l, thereis shown an antenna 5 connected toground I through the primary winding 6 of a radio-frequency transformer,the secondary winding 1 of which, tuned by afvariable condenser 8, isconnected'at one polnt'to the filament of the vacuum tube 9 in the firstradio-frequency amplifying stage and at another point to the grid ll ofthis vacuum tube. The output circuit of this vacuum tube extends fromthe filament system, through a highvoltage battery "13, a milliammeteri0, primary winding ll of a second radio-frequency transformer to theanode or plate ll of this vacuum tube. In order to neutralize theinherent capacity between the grid II and the plate It, and thereby toprevent oscillations, a neutralizing winding i9, electromagneticallycoupled to winding II, and neutralizing condenser 3 are employed in themanner described in the U. S. patents to Hazeltine, Nos. 1,489,228 and1,533,858. A second stage of radio-frequency amplification including thevacuum tube i5 neutralized by cooperation of coil 26 and condenser 4,like the first stage, comprises the secondary winding it of thelast-mentioned radio-frequency transformer tuned by a variable condenserl'i connected between the filament system of the vacuum tube II and ,thegrid l8 thereof. The output circuit of this vacuum tube also includesthe highvoltage battery 13 and a primary winding of .a secondradio-frequency transformer, while the secondary winding 2! of thistransformer tuned by a variable condenser 22 is included in the inputcircuit of a third stage of radio-frequency amplification which includesvacuum tube 23. The inherent capacity eflective between the electrodes24 and 25 is neutralized by a network including the neutralizingcondenser 28 and the neutralizing winding 29 as described in thementioned patents. The output circuit of the vacuum signal interceptedon the antenna I.

band of suitable size may be are given herewith. It

- its plate or anode 35 directly connected together to comprise ineffect a single anode.-

The three-stage amplifier functions in a manner well known in the art toamplify the incoming 15 The output circuit of the rectifier 33 includeswhat may be termed a rejector" circuit for stopping radiofrequencycurrents which have passed through the rectifier, and consists of anetwork including 20 a resistance 34 and a bypass condenser 31,connected between the anode I! and the filament 38 of the rectifier. Theoutput circuit'of the rectifier is coupled to the input circuit of anaudio-frequency amplifying vacuum tube a 95 through anaudio-frequency-pass filter including a fixed condenser 40' and animpedance 4i connected between the filament 42 and the grid 0 of thisvacuum tube. The output circuit of this amplifier is connected betweenthe filament l2 and plate 44 through the high-voltage battery "B" andthe primary winding ll of an audiofrequency transformer the secondarywinding 4' of which is connected .in the input circuit of a, secondaudio-frequency tube 41, while a resistance 48 connected across thewinding It serves to give the audio amplifier substantially uniformamplification over the desired frequencyrange. Instead of employingresistance II, a closed copper placed around the transformer winding soas to be electromagnetically coupled thereto. A loud speaker or otherreproducing device II, or if required, a coupling device for a telephonesystem, is connected in the output circuit of the in: audio- 5 frequencyamplifying tube 41. It is presumed that adequate precautions againstundesired electromagnetic coupling between the various radiofrequencycoupling transformers are included in all of the arrangements hereindisclosed. 50 In accordance with one of the main features of my saidcopending application Serial No. 203,879, means are provided to controlautomatically the degree of amplification effected in the radiofrequencyamplifying stages. These means in-" clude a resistance 5|, connectedbetween the filament 38 and the anode 35 of the rectifier, through whichthe pulsating rectified or converted current flows, thereby developing anegativevoltage at terminal 52 with respect to the cathode 38.

stage. Impedance 5:, together with blocking condenser 54, is effectiveto filter out and reject any audio-frequency currents which otherwisemight be present in the conductor 36.

To complete the description of the system illus trated in Fig. 1 certaindesign data or constants should be understood. however, that these, aswell as all other constants appearing in the present specification, arementioned merely by way of example in describing certain specificembodiments which in practice audio frequencies.

frequency stages indicated by the vacuum tubes 9, l5 and 23. Thisamplified signal voltage is then rectified by the rectifier 33, and therectified pulsating current is successively amplified by the audioamplifying stages including vacuum tubes 33 and 41, after which it maybe reproduced as sound by the loud speaker 50. When the rectified orconverted signal current flowing through the resistance 5| is greaterthan a predetermined value, there is developed at the terminal 52sufficient negative biasing voltage which in turn is impressed, throughthe conductor 36, upon the grid ll of the vacuum tube 9, to reduce theamplification of this tube. It will be apparent that as the magnitude ofthe rectified current flowing through resistance 5| decreases, thevoltage at terminal 52 becomes less negative, and the nega-,

tive biasing voltage impressed upon the grid II also diminishes so thatthe vacuum tube 9 effects an increased degree of amplification. In thismanner, the radio-frequency voltage applied to the input of therectifier is maintained at a nearly constant predetermined value, andthe volume of the reproduced signal is substantially uniform under allconditions. The degree of volume of the reproduced signal is thendetermined by adjustment of rheostat 49 which controls the heatingcurrent in the filament 42 of the first audiofrequency amplifying tube39. The neutralization of the grid-plate capacity of the radiofrequencyamplifying tubes is particularly valuable in that it allows an increasein the effectiveness of the amplification control, because suchneutralization prevents radio-frequency energy from pasting through thegrid-plate capacity of the tubes. Thus the relay action of the tubes isalmost entirely subject to the control by grid bias voltage.

The time required for operation of the control system would ordinarilybe determined by the lowest audio-frequency modulation which must bereproduced. Fading, for example, might be considered a form ofmodulation; the frequency of the rise and fall of signals due to fadingbeing the frequency of modulation. If this frequency of modulation beincreased sufficiently, 'the effect will be audio-frequency modulation.

It will thus be seen that if the automatic control attained by theprecent invention be allowed to respond too quickly, it will tend tosmooth out the desired modulation of the signals at the lower Hence, atime constant of operation is chosen which will be greater than theperiod of the audio frequencies which the system is intended to amplify.This time constant of the control circuit is equal to the product of theseries resistance and the shunt capacitance of the grid bias circuit,represented in Fig. 1 by resistance 53 and capacitance 54. However,since the time constant can always be reduced to a value equal to theperiod of the lowest modulation frequency, it may readily be set to meetthe requirements of nearly any special case which may arise. For.example, a value of two million ohms resistance and of 0.1 microi'aradcapacitance gives a time constant of one-fifth of a second, which doesnot appreciably affect the modulation at frequencies above five cycles.While this constant is less than required from the 5 point of view ofsatisfactory audio-frequency quality in the reproduction of music, thereappears to be no need for more rapid control under the conditionsusually encountered. The use in this connection of condensers of largecapaci- 10 tance, such as one-tenth microfarad, likewise introducesanother convenience in that the con- I densers may also serve to by-passradio frequencies in order to prevent undesired coupling between thedetector circuit and the first radiofrequency amplifying tube because ofsome impedance common to those two portions of the apparatus. I

There are advantages attending the use, in connection with theinvention'of my said copend- 2o ing application, of the two-electroderectifier.

circuit typified by Fig. 1, which may not be apparent from the foregoingdiscussion. It is impossible to overload this type of rectifier, and therectified output voltage is directly proportional to the appliedalernating signal voltage when this voltage is large, say over twovolts. The control system in the circuits of the figures referred torequires a large operating voltage, say ten volts, so that the lattercondition of large signal ac voltage is realized. No such simplerelationship is possible in a three-electrode detector whose rectifiedoutput never exceeds a limiting upper value, and is never proportionalto the applied voltage. except over a very small range of vol*- ages.This distinction will be seen from Fig. 2, where the abscissas A. C.represent the alternating signal voltages, and the ordinates D. C."represent the rectified output voltages. It is well known that thelinear curve is much more desirable when minimum distortion of amodulated signal is desired, and it will be observed from Fig.

2 that the preferred type of curve is obtained from the two-electroderectifier.

The three-electrode detector is useful for relg5 atively small appliedvoltages, and the rectified output voltage is then approximatelyproportional to the square of the applied voltage, 1. e., to the powerassociated with the applied voltage. For this reason the rectifiedvoltage increases with the carrier wave modulation. When such a detectoris used in the control system, the total power from the radio-frequencyamplifier is maintained at a substantially constant level, the amplitudeof the carrier wave being decreased in the presence of modulation. It isdesirable to maintain the carrier wave at a constant amplitude at theoutput of the amplifier. and this is accomplished by the two electroderectifier as shown in Fig. 1. The control system maintains constant 00the average signal amplitude which is equal to the carrier waveamplitude and independent of the degree of modulation.

t will be observed that in a system employing a. two-electrode rectifiersuch as represented by valve 33 of Fig. 1, the control bias voltage isindependent of the 3" batteryvoltage.

In the foregoing description, a tuned radio receiver of the neutralizedtype has been used as an example. It should be pointed out, however,that the present invention may be employed with equal effectiveness inany radio receivers or carrier-1 cially applicable to receivers of thesuperheterodyne type.

The behavior of the diode detector of Fig. 1 will now be explained inmore detail, with reference to Figs. 3 and 4. In Fig. 3, the diodedetector 55 comprises an anode I8 and cathode II. The signal voltage Eis supplied from the terminals of coil 58, and is applied to thedetector through condenser C. This condenser m be called a "blockingcondenser" because it prevents any electric charge on the anode 56discharging through any other path except the high resistance leakagepath R. Condenser C has a value such that its impedance tocarrier-frequency currents is much smaller than that of the tube 55 withR in par allel; but also such that its impedance to modulation-frequencycurrents is much larger than that of tube with R in parallel. The formercondition allows E, to be applied to the detectorterminals without loss;the latter condiion allows a rectified voltage E. across condenser C tofluctuate in accordance with modulation of the signal.

Curve (I) of Fig. 4 shows the relation between instantaneous current Iiand instantaneous voltage El at the detector terminals. The curve shownis a 3/2-power curve, which is the theoretical relation of the vacuumtube. This relation is most closely attained in tubes having auni-poten'ial cathode heated by indirect means, but the particular shapeof this curve does not materiallv affect the results described herein.It is only necessary tha the detector have practically unilateralconductivity.

The following theoretical discussion will serve to establish moredefinitely the mode of operation of the detector in the presence of amodulated carrier signal.

When there is no signal voltage E5, .ho anode voltage El is held at ornear that of the cathode by the leakage path R. Consider a sine-wavesignal applied to the detector, of the form Es=E 8111 out where E0represents the maximum vol'age of the wave; a) is 211' times thefrequency of the signal wave; and t represents time.

The first cycle produces a voltage swing E1 shown by curve During thepositive half of the cycle, the current I1 reaches a high value as shownby curve (21)). The condenser C is thereby charged negatively at anaverage rate indicated by the shaded area under curve (2b). This shadedarea is equal to the area within curve (2b). As the condenser receivesan increasing negative charge, the condenser bias voltage E. grows inmagnitude, and the positive swing of E1 is decreased both in amplitudeand duration, as indicated by curve (to). The charging rate is therebyrapidly decreased as indica'ed by curve (3b). Thecurrent curves (2b) and(3b) are drawn on a time base as indicated by the broken line arcsextending from the corresponding voltage waves. The relation between thebias voltage E. and the average rate of charge I. is shown by curve (4).The charging rate It in curve (4) obviously approaches zero as the biasvoltage E- approaches the signal peak voltage E0, because then therewould be no positive swing of the anode voltage E1. I

During this process, the average rate of discharge Ia, through R, issteadily increased as indicated by curve (5) and the equation When thecharging rate L and the discharging rate 14 reach equality, the biasvoltage on con-- stated above that the impedance of condenser C tomodulation frequency currents must be much greater than that of the tubewith R. in parallel.

It is apparent that Fig. 4 illustrates the determination of therectified voltage Ea for only one a value of signal peak voltage E0. Asimilar determination can be made either graphically or experimentallyfor various signal voltages, and the rectified direct current outputplotted against the u alternating current signal as in Fig. 2. Thisdetermination has been made both graphically and experimentally for anumber of different practical cases, and the results can be summarizedasfollows: When the signal voltage is relatively large, the rectifiedvoltage E. is only slightly less than the signal peak voltage E0, as inthe example of Fig. 4. This near-proportionality between the rectifiedoutput and the signal, is a linear relationship; and this is a kind oflinear detection". This mode of operation may also be called peakdetection, in order to distinguish from other types of linear detection.Peak deteotion it is noted, does not require a linear curve ofinstantaneous current and voltage, since curve (I) of Fig. 4 is notlinear, but is a 3/2-power. The re- 40 quirements for peak detection arequite different, as will be outlined below in more detail.

The first prerequisite of peak detection is a rectifier'whoseconductance in one direction is 5 very much smaller than that in theother direction. This is very well satisfied by the diode vacuum tube,whose conductance in the reverse direction is-zero, except for capacitycurrents. This is shown by curve (I) in Fig. 4.

The second prerequisite of peak detection is a leakage path or loadwhose conductance is very much smaller than that of the rectifier in thedirection of greater conductance. This is shown by curve 5) as comparedwith curve (I) in Fig. 4. The extent to which this relationship issatisfied determines the degree to which the rectified voltage Eaapproaches the peak voltage E0. It is apparent that, in the limitingcase of zero leakage current, curve (5) would coincide with thehorizontal axis and En would equal E0. It is also apparent thatcondenser C must be made sufliciently small to prevent additional audiofrequency'ourrents through this condenser, if the detector is to followthe envelope of a signal modulated at 65 audio frequencies.

The third prerequisite of linear detection is a signal voltagesufilciently great to extend beyond the curvature term (denominator) isincreased.

the magnitude of the leakage or load' conductance.

The transition region is very small and relatively unimportant in thecase of diodes having indirectly heated equipotential cathodes, but issomewhat greater in the case of diodes havin filament cathodes directlyheated by direct current.

The following rule, which has been derived on logical assumptions,expresses approximately the minimum value of peak signal voltagerequired in most cases to extend beyond the\ transitionregionofcurvefl):

d1,- 1) 2 as,- R

The first and second derivatives in the equation are those of curve (I)at its intersection with curve (5) intersection need not occur exactlyat the origin, and frequently does not in practical cases.) The term l/Ris, of course, the conductance of the leakage path or load. This valueof E0 is smaller when R is increased or when It is not within the scopeof this application to describe the derivation of this rule.

The initial value of signal voltage required to give substantiallylinear detection in practical circuits ranges from 0.1 ml volt, and maybe obtained from the above rule or by experiment. In the case of amodulated carrier, the carrier volt age must be much greater than thisinitial voltage because the signal voltage is at times much smaller thanthe average or carrier voltage. For example, the valueiof 2,,voltsmentioned above 1 was the computed carrier voltage required to give asignal always greaterthan 015 volt when modulatedupto%.r a I w when theabove prerequisites of peak detection are met in practice, as in theautomatic volume control receiver of Fig. 1,. the performance of thedetector is ideal. The rectified voltage closely follows the modulationenvelope of the signal voltage. and therefore the: modulation wave formis not distorted in the process of rectification. (The contrary is trueofthe common square-law detectors, and of any ordinary detectors whenoperated at saturation or overload levels) The diode detector operatedto give peak detection does not have any important overloading effectsbecause the peaks of the waves never become greatif; positive; in fact,its operation is somewhat better at'higher signal voltages, withinreasonable limits.

li'ig. 5 is the circuit diagram of a rectifying and indicating systemwhich takes advantage of peak detection and other refinements and whichhas been found eminently satisfactory for many purposes. It is usedprincipally to measure sinusoidal'alternating voltages oi! commercial,audio or radio. frequencies... Electrical constants will be given whichhavebeen employed in practice, but

tive filament terminal, and partly neutralizes the space charge aroundthe filament, without serving,any other function. Tube is a directcurrent amplifier in which the usual functions of the grid and plate areinterchanged, the plate servmg as control electrode and the grid asoutput electrode.

The unknown sinusoidal voltage to be measured is applied to the inputterminals. The rectifier is thrown in operation by switch 59, andunknown voltage is applied to the rectifier through condenser C (0.5mfd.) which has a neg ligible impedance at frequencies of voltages to bemeasured, but prevents the flow of direct current 1,0 between inputterminals and rectifier. Resistor R (5 megohms) is connected between theanode and the positive filament terminal, and serves to supply about onemicroampere initial anode current for tube 60. The rectifier currentflows through R and builds up at point 52 a rectifier voltage nearlyequal to the peak of the unknown voltage. v

The rectified voltage is applied to resistor 8i (1 megohm) and condenser62 (0.1 mfd.) which filter out the alternating current component andsupply a steady negative rectified voltage to the amplifier tube 63.Hence, this arrangement may properly be called a fiuctuation-rejector;.it tends to prevent over-loading of amplifier 63 by pre-' venting thealternating current component of the rectified voltage from beingimpressed thereon.

Battery 64 (9 volts) supplies the space current of tube 63. "Resistor I8(22,000 ohms) is connected in series to make the total output resistanceof tube 63 more nearly uniform thereby decreasing any distortion in theamplifying proc- .ess.

The microammeter 10 (100 microamperes full scale) carries the outputcurrent (about 400 microamperes) together with a nearly equal balancingcurrent through resistor 19 (6200 ohms). Rheostat 11 (2500 ohms) ispermanently adjusted to correct the slope of the calibration curve, aswill be described below in more detail, but does not affect the currentbalance existing 40 when switch 59 is disconnected. The meter isprotected by switch 16 when not in use. The balancing current iscontrolled by'voltage divider I5 (400 ohms).

The filaments are heated by battery 68 (6 45 volts). The rheostat 61 (2ohms) is so adjusted that voltmeter 69 reads 5.5 volts. The filamentvoltages are further decreased by resistors and 66 (each 6 ohms). Thelatter resistor 66 is so connected that the control electrode (plate) of60 tube 63 receives a slight initial negative bias through tube 50 andresistor GI. Resistors ll, 12, 13 and I4 (respectively 300, 210, 30 and10 ohms) serve to divide the voltmeter voltage in required parts foroutput and balancing circuits. 66 The filament battery therefore servesalso to furnish the grid bias voltage and balancing current.

Because of the high resistance external circuits of tubes 60 and 63, theperformance varies very 60 1 little with tube variations, and batteryvoltages need not be maintained precisely-constant.

The essential operation of the vacuum tube voltmeter of Fig. 5 will bedescribed with reference to the curves of Fig. 6, which show the rela-65 tions between the input alternating current voltage and theamplified-rectified current change in meter 10. The full scale of thismeter microamperes) is not shown because the curves are simply linear athigher values.

As originally constructed, rheostat 11 was absent, and the initialreading of meter 10 was adjusted to zero by voltage divider 75. Thecalibration curves then secured were like the dotdash curve (a). in thatthe meter readings did 75 ary with triode detectors.

rent voltages. It was noted that two corrections were required, one forthe curvature near the origin, and another for the slope of the linearpart of thecurve.

In a subsequent design, correction was made for the curvature at theorigin by supplying an excess balancing currentof 2 microamperes inmeter 10, and the slope was decreasedto the correct value by thepermanently adjusted rheostat 11. The rheostat I1 is therefore asensitivity adlusting means. The final calibration curve (b drawn in asolid line, then shows an exact correspondence between meter current andinput voltage, except at voltages below 5% of full scale. The result isa tube voltmeter which requires no reference to a calibration curve andis very accurate formeasuring sinusoidal voltages from 0.5 to volts(Root-mean-square values). The input circuit has such a high impedancethat the source of the unknown voltage mayhave as high as 3000 ohmsimpedance without causing any important error'of calibration.

Because of the low filament temperatures and non-critical circuitvalues, there is only a minute required to reach temperatureequilibrium. Thereafter, ,no readjustment is required during hours ofuse. An excessive overload does not injure the sensitive meter I0,because the plate v current ,of tube 63 cannot fall below zero andployed with equal effectiveness in such circuits.

Tube 80 is a screen-grid tube in the last stage of a multi-stageradio-frequency amplifier. Tube 8| is a triode with grid and plate tiedtogether, so as to be equivalent to a diode, and is used to obtain peakdetection. Tubes 82 and 88 are triodes used in the first two stages ofthe audiofrequency amplifier. Since the diode detector does not alsoamplify, it is necessary to employ one more audio-frequency stage thanis custom- The tubes shown all have indirectly heated cathodes, theheating circuits being omitted from the diagram. Like Fig. 1, thisreceiver also has automatic volume control.

The voltage for the screen-grid of tube 80 is supplied by battery 88.The plate circuit includes battery I I8 and auto-transformer 04. Thetransformer is broadly resonant over the broadcastband, and the primaryis shunted by resistor 80 to make the response more nearly uniform. Therectifier II receives the secondary voltage of transformer 80 throughcondenser C (50 11441.). Resistor R (0.2 megohm) is the leakage pathbetween anode and' cathode. Resistor 81 (0.25 megohm) and condenser 80(250 a t.) serve to reject the radio-frequency component, but to passthe audio-frequency and direct current components of the rectifiedvoltage to the grid of tube 82. The grid, or input, circuit of tube 82operates as a voltage responsive device having a tubes 82 and 03.

aoiasso very high input impedance and little or no input current fiow. i

j An automatic volume control arrangement supplies automatic grid biasvoltages to the various tubes as follows: An initialanegative voltage 5is applied to the entire detector circuit by battery 05 (3 volts). Addedto this bias is a second negative component due to the rectifiedvoltage.

across R. The second component serves to re duce the gain of certaintubes in the presence of 10 a strong signal. The total bias voltage isapplied to the grid of tube 02 through the radio-frequency reiectorcircuit 01, 88, and then also to the grids of the radio frequency tubes,preceding tube 00, through the audio-frequency rejector circuitincluding resistor 88 (0.5 megohm) and condenser 80 (0.01 to 0.1 mid.)Since the radio-frequency tube 00 is required to'maintain a fairly highgain, it is supplied with a smaller bias from a center tap 'on R,through the radio-frequency and audio-frequency reiector circuitincluding resistor (0.5 megohm) and condenser 01 (0.01 to 0.1 mid.).

The first two audio-frequency stages include Condenser II serves to sup-26 press any residual radio-frequency fluctuations accompanying theamplified output of the detector. Battery 88 supplies the space currentof tube 82, and battery 96 the grid bias of tube 08. Condenser 04 andresistors 02 and 05 are a re- 80 sistance coupling network between tubes82 and The latter resistor has a variable tap for use as a volume levelcontrol, in conjunction with the automatic volume control.

It should be understood that the element values as expressed inconnection with Fig. 7 and with succeeding figures are given merely toindicate values which will give good results: and should not beconstrued as limitations upon the invention.

Fig. 8 is a partial circuit diagram of another receiver employing bothpeak detection and automatic volume control. The tubes have indirectlyheated cathodes. Screen-grid tube I00 is in the lastradio-frequencyamplifier stage. Tube 15 I IIII is a ,triode used as adiode peak detector, or rectifier, having two terminals, namely theanode and the cathode, the grid being used as anode, and the plate beingtied to cathode and used as an electrostatic shield against anyelectrostatic 60 coupling which would otherwise exist between at leastone of the inner electrodes and other parts oi the circuit. Tube I02 isin the first audio-frequency amplifier stage.

Tube I00 has a screen voltage supplied by battery I03 and plate voltagesupplied by battery I04. An inductive radio-frequency transformer I05couples the radio-frequency amplifier to the detector, and is designedto give nearly uniform response over the broadcast band.

The detector circuit of Fig. 8 is an improvement over Fig. 7 in that theinductive transformer I00 permits the condenser C to be inserted in thecathode lead instead of in the anode lead of the diode detector. Thecondenser C 06 is connected in series between the cathode of therectifier and the transformer I08 which is a source of alternatingvoltage for the rectifier. The required radio-frequency voltage isthereby impressed between anode and cathode, but radio- 10 frequencycurrents are confined to the detector tube and condenser C, and are.nearly absent from all other parts of the detector circuit. Condenser C(100 nd.) and resistor R (50,000 ohms) serve the some purposes describedabove. rte-.75

sistor R is a leakage path across the rectifier, this leakage pathhaving a high and substantially uniform impedance to modulationfrequency and direct current and being connected between the lower endof the secondary winding of trans- .i'ormer I05 and ground, throughbattery H0.

The somewhat lower value of R, as compared with thatin the precedingcircuits, serves the added purpose of increasing the anode current andthereby-making the response of transformer I05 more uniform over thebroadcast band. There is not associated with the rectifier any leakagepath of lower resistance than R.

' In the arrangement of Fig. 8, the cathode terminal of the diodedetector is required to be grounded (which is accomplished through battery H). The condenser C is connected between the detector cathode andthe lower end of the secondary coil of transformer I05. Since thisungrounded secondary coil is principally an inductance, its resistanceis low in comparison with that of R. The alternating voltage to berectified is induced in this secondary coil from the primary coil oftransformer I05. The upper end of the secondary coil is connected to theother terminal (the anode) of the rectifier.

As in the case of Fig. 7, initial and rectified grid bias components arefurnished to the radiofrequcncy tubes by battery H0 (3 volts) anddetector NH. The grid bias for the radio-frequency tubes preceding tubeI00 is conducted through the audio-frequency and radio-frequencyrejectcr circuit including resistor I 06 (0.5 megohm) and condenser I01(0.01' to 0.1 mfd.). vA smaller grid bias is supplied to tube I00 from acenter tap on R, through the audio-frequency and radio-frequencyrejectcr circuit including resistor 108 (0.5

megohm) and condenser I09 (0.01 to 0.1 mfd.).

The audio-frequency amplifier is a voltage-responsive device having inits input circuit a voltage divider I'IB. The impedance of theamplifier,

including the voltage divider, is much higher than that of the leakagepath R. The audio-frequency amplifier can be connected either to theradio detector IM or to the phonograph pick-up 114, by throwing switchII3 to (Ra) or (Ph) respectively. In the former case, theaudio-frequency output of the detector is passed through the addedradio-frequency, or carrier-frequency, rejector circuit includingresistor III (0.25 megohm) and condenser H2 (100 ith), in order tocompletely remove radio-frequency fluctuations. This latter circuit isalso a direct-current rejectcr circuit because of the condenser II 5.The pure audio-frequency output is then applied through condenser I I5(0.005 mid.) to the voltage divider H6 (1 megohm). The variable tap onH6 to the grid of the audio amplifier serves to control the volume levelof the audio-frequency amplifier output, when used with either radio orphonograph excitation. -The'battery II'I supplies the required grid biasto audio-frequency tube I02. The remaining side, or terminal, namelythecathode, of the input circuit of the audio amplifier is required to begrounded, as shown. Also, the grid of the audio amplifier is connected(through elements II I, H5 and I I6) to the junction of condenser C andthe secondary coil of transformer I05. Hence, the audio amplifier, orvoltage-responsive device, is connected in parallel with condenser C andresistance R.

Fig. 9 is a partial circuit diagram of a radio receiver-employing peakdetection and automatic volume control, in which the detector circuitarrangement diiiers somewhat from the previously described arrangements.The tubes have indirectly heated cathodes. Screen-grid tube I20 is inthe last radio-frequency amplifier stage. Tube I2'I is a trlode used asa diode peak detector, the grid being umd as anode, and the plate being-5 tied to cathode and to ground and used as an electrostatic shieldagainst any electrostatic coupling which would otherwise exist betweenat least one of the inner electrodes and other parts I of the circuit.Tube I22 is in the first audio- 10 frequency amplifier stage.

Tube I20 has a screen voltage supplied by bat tery I25 and plate voltagesupplied by battery I26. A tuned .inductive radio-frequency transformerI21 couples the radio-frequency amplifier to the detector. The primary(left-hand) coil of this transformer is in the output circuit of tubeI20. The secondary (middle) coil is tuned to resonance by variablecondenser I28. In order to achieve certain advantages, the detector isnot connected to the secondary coil, but to a tertiary (right-hand) coilcoupled to the secondary.

The tertiary coil conveys to the detector only about one-third of theentire secondary voltage,

thereby preventing excessive damping of the 25 connecting the condenserC in the cathode return lead of the detector circuit (as in Fig. 8).,Condenser C (250 ar.) and resistor R (0.1 megohm) serve the samepurposes described above.

In Fig. 9, the radio-frequency tubes are supplied with initial grid biasvoltages in a customary manner, indicated for tube I20 by seriesresistance I24 and radio-frequency by-pass condenser '35 I23. Anadditional automatic grid bias is supplied fromthe rectifier voltage ofthe detector. The. radio-frequency tubes preceding tube I20 receive theentire automatic bias through the radio-frequency and audio-frequencyrejectcr circuit including resistor. I29 (0.5 megohm) and condenser I30(0.01 to 0.1 mfd.). Tube I20 receives a smaller additional bias throughthe radiofrequency and audio-frequency rejectcr circuit includingresistance I3I (0.5 megohm) and condenser I32 (0.01 to 0.1 mfd.).

The audio-frequency output of the detector is passed to the detectoroutput circuit through the radio-frequency rejector circuit includingresistor I33 (0.25 megohm) and condenser I 34 (250 e), in order tocompletely remove radio-frequency fluctuations. The pure audio-frequencyoutput is then applied through condenser I35 (0.01 mfd.) to the voltagedivider I36 (1 megohm). The variable tap on I36 serves to control thevolume level of the audio-frequency amplifier output. The firstaudio-frequency tube I22 has any conventional grid bias provision, suchas a series resistance I38 and an audio-frequency by-pass condenser I31.

The radio receivers of Figs. 1, '7, 8 and 9 have proven eminentlysatisfactory, especially with reference to the absence of distortion inthe process of detection. The peak detection employed provides arectified output which follows closely 05 the envelope of the modulatedradio-frequency signal. The achievement of peak detection is aided bythe automatic volume control which maintains the detector voltage withinthe proper operating range, regardless of the applied signal strength.

In additionto the above improvements, a further refinement has beendevised which is a desirable modification for peak detection circuits.This refinement is directed to a filtering arrangedirect-currentcomponent,

ment located at the output 'of the tube; and is' especially useful inlaboratory studies, as it enables the audio-frequency behavior of a peakdetector to be determined precisely by measurements made with directcurrent meters and without audio-frequency modulation of theradio-frequency signal. This refinement willbe explained with referenceto Fig. 10. L

a Fig. lpa shows the detector circuit of Figs. 8 and 9, reduced to itsessential elements,-namely: a diode rectifier; a radio-frequency inputcoil; the condenser (land the resistor R. The radio'- frequency currentsflow through condenser C, but the audio-frequency and direct-currentrectified components fiow through the resistor R. The direct-currentload and the audio-frequency load on the detector are thereforeefi'ectively the same, and are equal to the resistance R. This is adesirable condition, because all the useful modulation of signals occursin the audio-frequency range, and the rectified automatic bias voltageslie in the sub-audio-frequency or direct-current range. This rectifiercircuit behaves alike in these frequency ranges. The audio -frequencyand direct current output components are not separated, however, andexist between the terminal represented by the arrow, and ground. Theseparation of these components requires additional circuit elementswhich generally detroy this condition of identity between direct currentand audio-frequency load.

The structure of Fig. 10b has, in addition to the elements of Fig.10a,-a low-pass filter in which The cut-off frequency Jc is located slightlyfurther reduces radio-frequency fluctuations in g the direct-current andaudio-frequency output, but has a negligible effect on thedirect-current or audio-frequency load impedance.

The filters in the succeeding Figs. 100 to 1071. possess advantages overthe filters of Figs. 10a and 10b in that they enable the direct-currentand audio-frequency components of the rectified current to be separated.While the filter of Fig. 10b is shown nowhere else in Fig. 10, it isunderstood to be equally applicable to the other circuits of Fig. 10.

Fig. 100 shows, in place of the simple resistor R in Fig. 10a, a networkR, R, L', C which is known to have an effective resistance equal to R atall frequencies when the equation of condition is satisfied, namely:

This network has the ability to separate rapid fluctuations of theaudio-frequency component from the relatively .very slow fluctuationsof'the the dividing frequencyebeing the resonant frequency of thereactiveelements L and C. Thus the network R', R1, L, C includeselements L' and C whose impedances vary oppositely with frequency; theseimpedance elements, or the impedance paths in which the elements areincluded, have equal impedances at the frequency at which they areresonant, which resonant frequency is lower thanthe desired audiofrequencies; and the total impedance of the network is equal to a purereinductance L.

aoiam sistance over awide range .01 frequencies includ-- in: audiofrequencies and direct current. The" audio-frequency component is shownas deliv-' ered from the secondary of an inductive transformer whoseturns ratio, NizNz, may have any 5 desired value. The audio-frequencycomponent exists between the terminals marked AF: and the direct-currentpotential exists between ground and the terminal marked DC.

Fig. 1011 is similar to Fig. 100 except that the 10 resistance R inparallel with the primary induetance is replaced by a resistance R(Na/N1) across the secondary coil, the result being the, same. Avariable tap on this resistance may be used as a volume level control ifdesired, without it any detrimental effect.

Fig. 10c is similar to Fig. 100, except that the resistance R across theprimary L is replaced by a transmission line of proper impedance acrossthe secondary coil.

Fig. 10! is an example 'of a rectifier circuit using a different type ofconstant resistance network R, R, L',- C. The direct-current com-.ponent and the alternating-current component both flow in one shunt pathR, L. The alter- 26 hating voltage, designated AF is taken across Thedirect voltage, designated DC is taken from the point between elements Cand R.

Fig. 10a is an inverted 'form of Fig. 10!, and 0 accomplishes the sameresult. In addition, a variable tap audio-frequency volume level controlis indicated to be used if desired.

Fig. 107:. employs a third form of constant resistance network R, L',L', C, C. It has points 36 of similarity with both Figs. 10! and 10g,and accomplishes the same result.

The direct-current output components from all the above circuits can beemployed to advantage as automatic grid bias for automatic'vol- 40 uni;Jontrol, after the manner of Figs. l, '1, 8 an The same equation ofcondition stated above holds for all cases of Figs. 10b to 10h. Thisutilization of constant resistance networks is not 45 only useful indiode rectifier circuits, but is equally applicable and often veryuseful in the output circuits of other rectifier circuits, and ofamplifier circuits. In all such cases there is the same advantage ofuniform behavior toward both direct- 60 current and audio-frequencycomponents of the rectifier or amplifier output.

Many other applications of the improvements outlined above will beapparent but need not be discussed herein. They have also been foundequally useful in superheterodyne receivers and in other specialexamples. Itis obvious that the radio-frequency amplifiers referred toin the above cases can be replaced by amplifier-arrangegejrts of thesuperheterodyne or other special What is claimed is:

1. In combination with a vacuum tube rectifier in which alternating anddirect rectified-current components are produced, said rectifier havinginput and output terminals and requiring for best operation an outputimpedance connected to said output terminals which is equivalent to apure resistance, a network connected to said output terminals which hasa total impedance substantially equal to a pure resistance over a widerange of frequencies including audio frequencies and direct-current, andwhich includes impedance elements whose impedances vary oppositely withfrequency, said impedance elements having-equal 76 impedances at afrequency lower than the desired audio frequencies, so that therespective audio frequency and direct current components are separatedinto the paths of said impedance elements.

2. In combination with a vacuum tube rectifier in which alternating anddirect rectified-current components are produced, said rectifier havingoutput terminals and requiring for best operation an output impedanceconnected thereto which is equivalent to a pure resistance over a widerange of frequencies including audio frequencies and direct current, anetwork connected to said output terminals which has a total impedanceequal to a pure resistance over said wide range and which includes twoimpedance paths the impedance of a first of which increases withincreasing frequency, and the impedance of the second of which decreaseswith increasing frequency, said paths having equal impedances at afrequency lowerthan the desired audio frequencies, so that the directand audio-frequency current components are separated. respectively, intosaid first and second impedance paths.

3. In combination, a circuit including a nonlinear impedance in whichalternating and direct rectified-current components are produced, saidcircuit having output terminals and requiring for best operation a loadconnected thereto which is equivalent to a pure resistance over a widerange of frequencies including audio frequencies and direct current, anda load connected to said output terminals, said load being theequivalent of a pure resistance over said wide range and having a firstimpedance path whose impedance increases with increasing frequency and asecond impedance path whose impedance decreases with increasingfrequency, said paths having equal impedances at a frequency lower thanthe desired audio frequencies, whereby the direct-current component ofthe output of said circuit flows through said first path and theaudio-frequency component through said second path.

4. In combination, a circuit including a nonlinear impedance in whichalternating and direct rectified-current components are produced, saidcircuit having output terminals and requiring for best operation a loadconnected thereto which is equivalent to a pure resistance over a widerange of frequencies including audio frequency and direct current, and aload connected

